1. Field of the Invention
The invention relates to the production of alkyl esters, i.e., biodiesel. More particularly, the invention relates to a method of producing biodiesel with supercritical alcohol. More particularly yet, the method relates to a multi-step process that removes excess water from the feedstock.
2. Description of the Prior Art
Chemically, vegetable oil and animal fats are known as triglycerides. A triglyceride is a glycerin molecule (C3H5(OH)3) with three fatty acid molecules attached to three hydroxyl groups and glycerin is an alcohol with three hydroxyl groups.
A fatty acid is a long chain hydrocarbon molecule, and is found in vegetable oils and animal fats. Stearic, palmitic, and oleic acids are examples of fatty acids. A fatty acid is also known as an ester. Sometimes one or more of the fatty acids are detached from the glycerin molecule. These are known as free fatty acids or FFAs. A glycerin molecule with a single fatty acid attached is known as a monoglyceride, while a glycerin molecule with two fatty acids is known as diglyceride.
Biodiesel, properly known as an alkyl ester, is the result of transesterification of glycerides in which the fatty acid molecules are removed from the hydroxyl group of the glycerin and attached to the hydroxyl group of a single headed alcohol. Methanol, ethanol, propanol, and butanol are examples of single headed alcohols. The resulting biodiesel is known by the alcohol from which it was formed. Alkyl esters made with methanol are known as methyl esters; with ethanol as ethyl esters; with propanol as propyl esters; and with butanol as butyl esters. Alkyl esters can also be formed by esterification of FFAs, or transesterification of glycerides. An FFA is esterified when it combines with an alcohol molecule. A water molecule is formed when an FFA is esterified. Esterification is desirable for certain feedstocks, especially those with a high percentage of FFAs.
The most common means of performing transesterification is with the use of a base catalyst. The most common catalysts are potassium or sodium based. Small-scale biodiesel production processes use sodium hydroxide or potassium hydroxide, commonly known as lye. Larger biodiesel producers use sodium or potassium methylate, which is essentially the metal dissolved in methanol. Base transesterification works best with feedstocks low in FFAs because each FFA molecule combines with a catalyst molecule and produces a molecule of soap, rather than an alkyl ester. The loss of yield to the production of soap is typically 5-10%. The transesterification process takes about 30-120 minutes. The soap then has to be removed from the product stream by means of some cleaning process and the catalyst in the byproduct neutralized.
The most common means of esterification is with a strong acid catalyst such as sulfuric acid. The reaction is much slower than base transesterification, taking 4-8 hours, and because of that, generally requires the use of large capacity tanks. Furthermore, the reaction is self-limiting, because the esterification process itself produces water and water retards the reaction. Multiple stages are required if the feedstock is sufficiently high in FFA content. For example, waste oil that contains 20% FFA must go through at least two acid stages in order to be processed by the normal base stage. The first stage may reduce FFA content to 5%, the second to 1%. The first acid stage in a case like this produces a mixture of methanol, water, and acid that can be drained off, because of a combined density greater than that of the oil. At a minimum, the water must be removed before the next acid stage can take place.
A troublesome fluid byproduct that needs to be removed is produced in this step of the process. Methanol dissolves in water, as does sulfuric acid. The acid esterification process mixes dry oil, methanol, and sulfuric (or another very strong) acid. It's circulated for some hours to keep it well mixed, then left to stand. Over some additional hour(s), if enough FFAs were esterified to produce enough water, the mix of water/methanol/acid will sink to the bottom where it can be drained off. The acid must then be neutralized or saved for later use in neutralizing the base catalyst and the methanol must be recovered from the water. It's always problematic; the water must be removed, because it will cause soap formation in the base catalyzed phase, and removing the water also removes the alcohol. Furthermore, acid esterification incurs greater expense at each step of the process: tanks much larger than the transesterification tanks are needed; acid must be added and later neutralized; and methanol has to be recovered from the water produced by the esterification.
An alternate means of esterifying FFAs and transesterifying glycerides is to put the FFAs or glycerides in the presence of an alcohol in a supercritical phase. A phase is supercritical when the pressure and temperature are above the critical point for the alcohol. For methanol, the critical pressure is 81 bar, or approximately 1190 psi, and the critical temperature is 239.6 degrees C., or approximately 463 degrees Fahrenheit. In a supercritical phase, liquid disperses evenly throughout its environment, as a gas does, thus eliminating the need to emulsify the alcohol in the feedstock.
Keiichi Tsuto, et al, in U.S. Pat. No. 6,288,251 teach a supercritical method of transesterifying glycerides, in which a virtually complete conversion of glycerides occurs in several minutes, without a catalyst, at molar ratios of 40 to 1 or more alcohol to glycerin (for glycerides), and at temperatures of 350 degrees C. (662 degrees Fahrenheit) and pressures of 40 MPa (6,000 psi).
Commercial acid esterification processes use strong acids, such as sulfuric acid, and an alcohol, usually methanol. The esterification process produces water, which stops the reaction. Multiple stages are required if the FFA percentage is high, and the water/acid/methanol has to be drained between stages. Additional acid and methanol must be added to complete the esterification process. This acid esterification process is much slower than the base esterification process, normally taking a minimum of 4 hours.
Production of biodiesel with supercritical alcohol is known. Using a supercritical process for the esterification of FFAs or transesterification of glycerides avoids many of the problems of acid esterification and base transesterification. No base catalyst is used, so there is no saponification of FFAs, and the byproducts are nearly pure glycerin and excess alcohol with traces of water. Shiro Saka discloses in U.S. Pat. No. 7,227,030 that the supercritical process tolerates higher levels of water in the feedstock than catalytic processes. Cleaning the fuel after processing is greatly simplified because there is no soap to remove. The process yield approaches 100%, because FFAs are esterified rather than saponified. Any feedstock can be used, even 100% FFAs, because the FFAs are esterified without a catalyst.
The conventional supercritical process must run at temperatures from 500 to 700 degrees Fahrenheit and pressures from 3000 to 6000 psi. These operation parameters require the use of expensive equipment. The ratio of alcohol to feedstock is extremely high. For example, approximately 1.5 gallons of methanol must be mixed with each gallon of FFAs or glycerides that are fed through the process. The processor must be large enough to hold 2.5 gallons of the mixture for each gallon of alkyl esters to be produced, and, thus, must be approximately twice the size of a reactor used with the normal ratios of alcohol to feedstock in the catalytic process. Furthermore, the alcohol must be separated out at the end of the process. It is, of course, much more expensive to remove 1.4 gallons of alcohol, rather than 0.1 gallon. The ratio of alcohol to feedstock is necessarily a molar ratio of oil molecules to alcohol molecules. Ethanol/propanol/butanol molecules are larger than methanol molecules, and thus, an even greater volume of higher alcohols is required when using these alcohols.
What is needed, therefore, is a method of producing biodiesel that requires less time than conventional methods and is less expensive to implement. What is further needed is a process that prevents FFAs from binding with glycerin during processing. What is yet further needed is a process that reduces the ratio of alcohol to feedstock required to obtain full conversion of glycerides to alkyl esters.